LTE (Long Term Evolution) Rel. 8 (Release 8) developed in 3GPP (3rd Generation Partnership Project Radio Access Network) employs SC-FDMA (single-carrier frequency-division multiple-access) involving a low PAPR (peak-to-average power ratio) and high electric-power use efficiency of a terminal apparatus (also referred to as UE) as an uplink communication scheme (refer to Non-Patent Literatures (hereinafter, abbreviated as NPLs) 1, 2, and 3).
In the uplink of LTE, both a data signal of PUSCH (Physical Uplink Shared Channel) and a control signal of a PUCCH (Physical Uplink Control Channel) are transmitted on a per-subframe basis (refer to NPL 1).
Each subframe includes two time slots. In each time slot, a plurality of SC-FDMA symbols and a DMRS (demodulation reference signal, also abbreviated as DRS) as one of the reference signals are time-multiplexed. FIG. 1 illustrates an example of a subframe configuration of a PUSCH to which normal cyclic prefix is applied.
Upon reception of a data signal on the PUSCH, the base station performs channel estimation using a DMRS. The result of channel estimation result is used to demodulate and decode an SC-FDMA symbol. In addition, in LTE-A (LTE-Advanced) Rel. 10 (Release 10), DFTS-OFDM (discrete-Fourier-transform spread orthogonal frequency division multiplexing), which is an extended version of SC-FDMA, has also become available. This is a method of dividing into two spectra the data signals of the PUSCH obtained as illustrated in FIG. 1, and mapping the divided signals to different frequencies, and thereby expanding the flexibility of scheduling without any change in the configuration of the PUSCH from that of SC-FDMA.
On the other hand, the control signal of PUCCH has a plurality of formats each having a subframe configuration different from that of PUSCH (refer to NPL 1). Any one of the formats involves a subframe consisting of two slots, and each slot includes a plurality of SC-FDMA symbols and a plurality of DMRSs. The control signal of PUCCH is demodulated on the basis of a channel estimation result using the DMRS similarly to the data signal of PUSCH.
In order to correctly decode information transmitted on the PUSCH and PUCCH, highly precise channel estimation by the DMRS is indispensable. In Rel. 10 of LTE-A, the DMRSs of different UEs connected to the single cell are transmitted while different cyclic shifts (CSs, equivalent to phase rotations) or orthogonal codes (also referred to as orthogonal cover codes (OCCs)) are allocated to the DMRSs. Accordingly, these DMRSs can be orthogonally multiplexed and orthogonally demultiplexed.
On the other hand, the DMRSs transmitted by different UEs connected to different cells cannot be demultiplexed. Therefore, in order to suppress inter-cell interference and to perform highly precise channel estimation, it is necessary to use different signal sequences having a small correlation between different cells.
Consequently, in Rel. 8-10 of LTE-A, 30 different base sequence groups mutually having low correlations are defined and are respectively allocated to cells. Moreover, in order to avoid causing uneven magnitude of interference, sequence group hopping that changes the sequence group used for every time slot is introduced.
The base sequence group refers to a group including a plurality of base sequences having a plurality of sequence lengths. Moreover, the sequence group refers to a group obtained by gathering a plurality of signal sequences obtained by cyclically shifting all the respective base sequences in one base sequence group. Therefore, allocating a base sequence group to a cell means the same as allocating a sequence group corresponding to the base sequence group to the cell.
FIG. 2 illustrates the feature of a sequence group allocated to each cell.
For example, when Ncell ID represents a cell-specific identifier ID (cell ID number), a sequence group corresponding to a number given by the following expression is allocated in an ns-th slot in the cell.
 [1]u=(fgh(ns)+fss)mod 30, ns=0˜19  (Expression 1)
In expression 1, fgh(ns) is a hopping pattern represented with a sequence group determined by a PN (pseudo-random) sequence having an initial value equal to an integer value obtained by truncating Ncell ID/30 after the decimal point, and Ncell ID is given by an integer of 0 to 503.
Offset fss is an integer determined by the following expression, and is provided to each cell.
                    (                  Expression          ⁢                                          ⁢          2                )                                                                      f          ss                =                  {                                                                                          N                                          cell                      ⁢                                                                                          ⁢                      ID                                                        ⁢                  mod                  ⁢                                                                          ⁢                  30                                                                              for                  ⁢                                                                          ⁢                  PUCCH                                                                                                                          N                                          cell                      ⁢                                                                                          ⁢                      ID                                                        +                                                            Δ                      ss                                        ⁢                    mod                    ⁢                                                                                  ⁢                    30                                                                                                for                  ⁢                                                                          ⁢                  PUSCH                                                                                        [        2        ]            
In expression 2, Δss is one of the integers of 0 to 29 and is indicated to a UE as a cell-specific parameter. The sequence group hopping can also be disabled (that is, setting fgh(ns)=0) on a per-cell basis or on a per-UE basis.
The following is found from the above.
1. In many cells included in a system, according to 504 pieces of different Ncell ID, 30 sequence groups are repeatedly used in different cells.
2. A sequence hopping pattern (fgh(ns)) is the same among 30 cells having continuous Ncell ID, and the hopping patterns of sequence groups are different between the cells having an interval of Ncell ID equal to or more than 30. A cell group with the same hopping pattern (fgh(ns)) constitutes a cell cluster.
3. In one cell cluster, offset fss varies from cell to cell in principle. In other words, this is equal to using patterns obtained by applying cell-specific sequence offset fss to a common sequence group hopping pattern.
FIG. 3 illustrates an example of sequence group hopping. FIG. 3 illustrates hopping patterns of a sequence group used in a cell cluster including Ncell ID=0 to 29. For the sake of simplicity, it is assumed that Δss=0 in all cells.
As illustrated in FIG. 3, a hopping pattern is common in one cell cluster, but a different sequence offset is given according to the value of Ncell ID. Therefore, in a cell cluster, a different sequence group is always used in a different cell. Moreover, hopping patterns themselves differ in different cell clusters. Therefore, in cells in different cell clusters, even if sequence groups overlap, causing an increase in interference, the interference does not continue over a plurality of slots.
The base sequence group allocated to each cell in this way is used, for example, for an uplink SRS (sounding reference signal) that is one of the reference signals and modulation of a PUCCH in addition to the DMRS.